In the development of electroluminescent elements utilizing organic materials (hereinafter referred to as organic EL element), the kind of electrodes was optimized for the purpose of improving the electron-injecting efficiency from the electrode and an element in which a hole-transporting layer of an aromatic diamine and a light-emitting layer of 8-hydroxyquinoline aluminum complex are disposed as thin films between the electrodes has been developed (Appl. Phys. Lett., Vol. 51, p. 913, 1987) to bring about a noticeable improvement in luminous efficiency over the conventional elements utilizing single crystals of anthracene and the like. Following this, the developmental works of organic EL elements have been focused on their commercial applications to high-performance flat panels characterized by self luminescence and high-speed response.
In order to improve the efficiency of such organic EL elements still further, various modifications of the aforementioned basic structure of anode/hole-transporting layer/light-emitting layer/cathode have been tried by suitably adding a hole-injecting layer, an electron-injecting layer and an electron-transporting layer. For example, the following structures are known: anode/hole-injecting layer/hole-transporting layer/light-emitting layer/cathode; anode/hole-injecting layer/light-emitting layer/electron-transporting layer/cathode; and anode/hole-injecting layer/light-emitting layer/electron-transporting layer/electron-injecting layer/cathode. The hole-transporting layer has a function of transporting the holes injected from the hole-injecting layer to the light-emitting layer while the electron-transporting layer has a function of transporting the electrons injected from the cathode to the light-emitting layer.
A large number of organic materials conforming to the function of these layered structures have been developed.
The aforementioned element comprising the hole-transporting layer of an aromatic diamine and the light-emitting layer of 8-hydroxyquinoline aluminum complex and many other elements utilize fluorescence. Now, the utilization of phosphorescence, that is, emission of light from the triplet excited state, is expected to enhance the luminous efficiency approximately three times that of the conventional elements utilizing fluorescence (singlet). To achieve this object, studies have been conducted on the use of coumarin derivatives and benzophenone derivatives in the light-emitting layer, but the result was nothing but extremely low luminance. Thereafter, the use of europium complexes was attempted, but it was unable to obtain high luminous efficiency.
The prior technical documents relating to this invention are listed below.                Patent literature 1: JP2002-352957 A        Patent literature 2: JP2001-230079 A        Patent literature 3: JP2001-313178 A        Patent literature 4: JP2003-45611 A        Patent literature 5: JP2002-158091 A        Non-patent literature 1: Nature, Vol. 395, p. 151, 1998        Non-patent literature 2: Appl. Phys. Lett., Vol. 75, p. 4, 1999        
The possibility of emitting red light at high efficiency by the use of a platinum complex (PtOEP) is reported in the aforementioned non-patent literature 1. Thereafter, it is reported in non-patent literature 2 that the efficiency of emitting green light has been improved markedly by doping the light-emitting layer with iridium complexes (Ir(ppy)3). It is reported further that optimization of the light-emitting layer enables these iridium complexes to show extremely high luminous efficiency even when the structure of an element is simplified.
In applying organic EL elements to display devices such as flat panel displays, it is necessary to improve the luminous efficiency and at the same time to secure the driving stability. The organic EL elements utilizing phosphorescent molecules of Ir(ppy)3 described in non-patent literature 2, although highly efficient, are not suitable for practical use because of their insufficient driving stability at the present time (Jpn. J. Appl. Phys., Vol. 38, L1502, 1999).
The main cause of the deterioration of the aforementioned driving stability is presumed to be the deterioration of the shape of thin film of the light-emitting layer in the structure of an element such as substrate/anode/hole-transporting layer/light-emitting layer/hole-blocking layer/electron-transporting layer/cathode or substrate/anode/hole-transporting layer/light-emitting layer/electron-transporting layer/anode. It is likely that the deterioration of the shape of thin film is attributable to crystallization (or cohesion) of thin organic amorphous films caused by generation of heat during driving of the element and poor heat resistance is due to low glass transition temperature (Tg) of the material in use.
It is described in non-patent literature 2 that a carbazole compound (CBP) or a triazole compound (TAZ) is used in the light-emitting layer and a phenanthroline derivative (HB-1) is used in the hole-blocking layer. Because of their high symmetry and low molecular weight, these compounds readily undergo crystallization or cohesion and suffer deterioration of the shape of thin film. Besides, their crystallizability is too high to allow observation of their Tg. Such instability of the shape of thin film of the light-emitting layer adversely affects the performance of an element, for example, by shortening the driving life and lowering the heat resistance. For the reasons described above, a difficult problem facing phosphorescent organic electroluminescent elements at the present time is their driving stability.
It is disclosed in the aforementioned patent literature 1 that a compound containing an oxadiazolyl group is used as a host material in an organic EL element comprising a host material and a phosphorescent dopant material in its light-emitting layer. An organic EL element comprising a thiazole or pyrazole structure in its organic layers is disclosed in patent literature 2. An organic EL element comprising a phosphorescent iridium complex and a carbazole compound in its light-emitting layer is disclosed in patent literature 3. An organic EL element comprising a carbazole compound (PVK), a compound containing an oxadiazolyl group (PBD) and an iridium complex (Ir(ppy)3) in its light-emitting layer is disclosed in patent literature 4. Ortho-metalated metal complexes and porphyrin metal complexes are proposed as phosphorescent compounds in patent literature 5. However, they face the aforementioned problem. It is to be noted that patent literature 2 discloses no organic EL elements utilizing phosphorescence.